The design alternatives available for subframing attachment systems include wood, steel, PVC/fibercement battens, or aluminum. In addition, there are also alternatives available for the use of either fixed system details that require supplemental shimming of the system to address any out of tolerance conditions in the substrate, or adjustable systems that use brackets that allow for internal adjustments to compensate for out of tolerance substrate conditions (see future posts and a link to ALLFACE Smart Fixing Systems in the product links section for more information).
So understanding the implications associated with the use of each of these subframing components allows the system designer to make an educated decision on appropriate material selection (based upon design and budget) and what the life expectancy for each will be.
For the sake of this analysis, we are assuming that the mechanics of the rainscreen system will follow suit with the physics of a rear ventilated rainscreen design. The intent of this system approach is not to keep moisture out of the cavity, but to limit the membranes exposure to moisture thru proper design. So how the subframing components respond to moisture is important for the longevity of the system, and sometimes the structural performance of the system.
So when considering products like wood battens, special care must be taken to assure that the elements are properly treated to prevent moisture related rot and warpage. As a minimum pressure treated lumber should be used, as well as full joint closures and cavity drainage elements.
The use of fibercement and PVC battens are typically only to provide sufficient cavity spacing (1/2 inch +/-), and are not intended to be structural elements. So for commercial and highrise applications, the decision really comes down to the use of steel versus aluminum subframing systems.
Conventional rainscreen design wisdom has gravitated towards the use of Aluminum subframing systems as the "material of choice" for various reasons. Notwithstanding, the long term benefits gained from its use far outweigh any additional initial costs that may be associated with its use.
The low weight and high strength, malleability, simplicity of fabrication, corrosion resistance and good ability to conduct heat and electricity are some of the most important characteristics of Aluminum. Also Aluminum is the most widely recycled product due to its abundant post consumer and post industrial stock available, its low melting point and limited energy required to recycle.
The density of Aluminum is approximately one third the weight of steel. So for sustainable design applications where the weight of the enclosure system can impact the sizing of the building structure or foundations, this can be an important factor to consider. But the weight of aluminum doesn't negatively impact the strength of the material as it relates to cladding systems where the limitation is actually with the spanning capabilities of the cladding material, and not the strength of the subframing.
But one important design factor to consider when using aluminum is the coefficient of thermal expansion of the material. Compared with other metals aluminum has a relatively large coefficient of linear expansion. So the system designer must take into consideration not only the thermal movements of the subframing, but the thermal movements of the cladding material at panel fastening locations. Otherwise an improper system design can lead to cracks at fastener locations, or cracks in the panels due to buckling or panel to panel contact.
As indicated by its position in the partial electromotive force series, Aluminum is a relatively reactive metal; among structural metals (only beryllium and magnesium are more reactive). Aluminum owes its excellent corrosion resistance to the barrier oxide film that is bonded strongly to the surface and if damaged reforms immediately in most environments. On a surface freshly abraded and exposed to air, the protective film is only 10 Angstroms thick, but highly effective at protecting the metal from corrosion.
The Swedish Institute for Metal Research into Corrosion has carried out open-air experiments with different untreated metals. These show the losses in weight of sheet metals with untreated surfaces after 8 years exposure for both inland and coastal (less than 1 mile from the coast) locations. In a mainland atmosphere of moderate saline content, the durability of aluminum is excellent. The following results were noted in these open air experiments:
Inland Location
Material : Weight Loss
Aluminium : 2g/m2
Copper : 31g/m2
Zinc : 61g/m2
Carbon Steel : 676g/m2
In strong saline atmospheres, it is possible that a small level of corrosion may appear on the surface. But generally the durability of Aluminum far exceeds that of either carbon or galvanized steel. The occurrence of salts, especially chlorides, in the atmosphere only slightly reduces this durability by comparison with other metals. The effects upon carbon steel are much more severe.
Coastal Location
Material : Weight Loss
Aluminium : 7g/m2
Copper : 57g/m2
Zinc : 133g/m2
Carbon Steel : 933g/m2
The average for the deepest corrosion on aluminum sheets after 8 years was 70μm (0.07mm) approximately 1/100 the loss of carbon steel and 1/10 the loss of galvanized steel (see zinc in the table).
So when considering the effects of corrosion on metals, the system designer must consider the two chemical processes involved: oxidation and reduction. The oxidation process takes place at an area known as the anode. The four essential components that are needed for a corrosion reaction to occur are an anode, a cathode, an electrolyte with oxidizing species, and some direct electrical connection between the anode and cathode. Although atmospheric air is the most common environmental electrolyte, natural waters, such as seawater rain, as well as man-made solutions, are the environments most frequently associated with corrosion problems.
Experience with the use of carbon and galvanized steel in rainscreen applications has shown that at areas where panels are fastened directly to the metal subframing, exposure to corrosive failure is concentrated at the exposed/cut edges of the steel elements and at fastening holes. So the system designer must take into account the potential for material density loss (oversizing of fastening holes) that can develop with steel subframing and what that means for the chance of future panel failures. Aesthetic issues also include corrosive streaking along the panel face due to the corrosion of steel subframing elements.
Similar problems have not been shown to be an issued with architectural grade aluminum.
Friday, July 10, 2009
Thursday, July 9, 2009
How does cavity depth affect the performance of a rainscreen wall?
The depth of the rainscreen wall cavity is an important design consideration. Cavity depth affects the free flow of air in the cavity, which in turn can effect moisture drainage. Cavity depth can also play an important role in the insulating properties of the rainscreen wall (if properly considered).
For residential construction, Canadian Building Experts recommend a minimum cavity depth of approximately 1/2inch. This is the rule of thumb minimum depth that has been determined will still allow the free flow of air in the cavity (and encourage cavity drainage away from the sheathing and membrane - assuming adequate flashing and guttering members have been designed). But this depth is precipitated upon the fact that the cladding application can utilize vertical battens attached directly to the sheathing. This type of approach is what we may see as a solution for low windload applications, single family and type V residential construction. This is what is commonly termed a "simple rainscreen" or "residential rainscreen" approach.
For Commercial construction, higher performing residential construction, and anything over 3 stories, we would anticipate the use of a deeper cavity. For steel stud construction, typical stud spacings occur at 16 inches OC. So a system designed to a 40 psf windload per ICC, should be sufficient for most project windload designs (that's a design windload in excess of 120 psf). That means that the subframing system must attach directly to the steel studs, and be sufficient to support the project design loads. In that case, we would expect to see a cavity depths in excess of 3 inches.
The deeper the cavity, the greater the potential for resistance to the forces of wind driven rain (negative pressure differential). But its not that simple, yet something that needs to be considered as part of a quality wall design.
For a pressure equalized rainscreen wall, the depth of the cavity is instrumental in the cavitation simulation and compartmentalized design. Cavity depth affects the volume of the wall compartments, which in turn impacts the sizing of venturi slots (ventilation baffles). Its not merely a function of drilling holes into perimeter extrusions or encouraging the free flow of air through panel joint constructions. Rather critical analysis of the how air flows into the PER cavity (and out it, i.e. the permeability of the membrane) will determine the effectiveness of this wall type.
For the rear ventilated rainscreen systems, several factors impact the design approach for addressing negative pressure differential in the cladding cavity. These systems require sufficient airflow from the base of the cavity to the top of the cavity. When you also increase the cavity depth, you reduce the potential for moisture transfer across this plane. Also, air and water testing has shown that if you also integrate a closed jointed design, you will limit the intrusion of water into the cladding cavity. This design feature can also act as a great deterrent to UV degredation of the waterproof membrane.
With modifications to regional energy codes and the move towards improved thermal envelope design, we are seeing the introduction of exterior grade insulations (min
eral wool) in the wall cavity to offer improved R values for the enclosure system. The system to the right for the Artesian Water Company in Delaware integrated an 11-1/2 inch deep adjustable system to support a ceramic tile rainscreen system. Over 4 inches of mineral wool insulation was integrated into the design to improve energy and acoustic performance of the wall. This is an added benefit that a rainscreen system offers over conventional construction (yet doesn't adversely impact the installed cost of the cladding). Groups like Lawrence Berkeley National Labs, Oak Ridge National Labs and the DOE are involved in assessing the use of high performance wall cladding with regards to the energy saving payback period that their use assumes. For projects where an integrated design approach is taken for the design of the enclosure systems and MEP systems we would anciticipate a very short payback period for the use of such systems.
For residential construction, Canadian Building Experts recommend a minimum cavity depth of approximately 1/2inch. This is the rule of thumb minimum depth that has been determined will still allow the free flow of air in the cavity (and encourage cavity drainage away from the sheathing and membrane - assuming adequate flashing and guttering members have been designed). But this depth is precipitated upon the fact that the cladding application can utilize vertical battens attached directly to the sheathing. This type of approach is what we may see as a solution for low windload applications, single family and type V residential construction. This is what is commonly termed a "simple rainscreen" or "residential rainscreen" approach.
For Commercial construction, higher performing residential construction, and anything over 3 stories, we would anticipate the use of a deeper cavity. For steel stud construction, typical stud spacings occur at 16 inches OC. So a system designed to a 40 psf windload per ICC, should be sufficient for most project windload designs (that's a design windload in excess of 120 psf). That means that the subframing system must attach directly to the steel studs, and be sufficient to support the project design loads. In that case, we would expect to see a cavity depths in excess of 3 inches.
The deeper the cavity, the greater the potential for resistance to the forces of wind driven rain (negative pressure differential). But its not that simple, yet something that needs to be considered as part of a quality wall design.
For a pressure equalized rainscreen wall, the depth of the cavity is instrumental in the cavitation simulation and compartmentalized design. Cavity depth affects the volume of the wall compartments, which in turn impacts the sizing of venturi slots (ventilation baffles). Its not merely a function of drilling holes into perimeter extrusions or encouraging the free flow of air through panel joint constructions. Rather critical analysis of the how air flows into the PER cavity (and out it, i.e. the permeability of the membrane) will determine the effectiveness of this wall type.
For the rear ventilated rainscreen systems, several factors impact the design approach for addressing negative pressure differential in the cladding cavity. These systems require sufficient airflow from the base of the cavity to the top of the cavity. When you also increase the cavity depth, you reduce the potential for moisture transfer across this plane. Also, air and water testing has shown that if you also integrate a closed jointed design, you will limit the intrusion of water into the cladding cavity. This design feature can also act as a great deterrent to UV degredation of the waterproof membrane.
With modifications to regional energy codes and the move towards improved thermal envelope design, we are seeing the introduction of exterior grade insulations (min
eral wool) in the wall cavity to offer improved R values for the enclosure system. The system to the right for the Artesian Water Company in Delaware integrated an 11-1/2 inch deep adjustable system to support a ceramic tile rainscreen system. Over 4 inches of mineral wool insulation was integrated into the design to improve energy and acoustic performance of the wall. This is an added benefit that a rainscreen system offers over conventional construction (yet doesn't adversely impact the installed cost of the cladding). Groups like Lawrence Berkeley National Labs, Oak Ridge National Labs and the DOE are involved in assessing the use of high performance wall cladding with regards to the energy saving payback period that their use assumes. For projects where an integrated design approach is taken for the design of the enclosure systems and MEP systems we would anciticipate a very short payback period for the use of such systems.
Friday, July 3, 2009
We ran across this photo on a flickr site in Seattle: http://www.flickr.com/photos/14361725@N07/2986359346/
Its a compelling photo visually, but not what you really want for a panel product if the end result is longevity and moisture control. And it's a great illustration of what happens when moisture isn't sufficiently controlled in a cladding system (especially with materials that aren't designed to be continually wet). And I'm sure its also probably an excellent example of someone being penny-wise but dollar foolish with the design of the system.
As part of any good cladding design, the end goal should be to control the forces that allow moisture to enter the cavity of the cladding and allow moisture to dry and exfiltrate the cavity. In the case above, it appears that edges of the panels are staying wet, causing the panels to absorb moisture and discolor.
Moisture attacks these surface edges through forces generated by gravity flow of water, capillary action and surface tension, or from an inward drive of moisture caused by negative pressure differentials in the cavity of the cladding. Also, if the cavity of the cladding is closed off or restricted from having sufficient ventilation, you may find yourself experiencing significant problems associated with condensation that occurs at the backside of your panel.
With wood and paper-based products where moisture can shorten the lifespan of your product, designing the system sufficiently will allow you to extend the serviceable life of the product. An accumulation of moisture at the backside of the panel can lead to panel bowing and warpage and ultimately failures at the fastening locations due to the stresses caused from the panel movement.
We find that one of the most underrated components of any exterior enclosure application are flashings and system terminations. You may address all of the factors mentioned above at the exterior face of the panel, but if there is a failure where it abuts a dissimilar material or where the system terminates, you could have significant moisture infiltration problems associated with the installation. That is why we feel it is critical to consider the use of products where the panel supplier is intimately involved with the design and supply of the attachment system. Who knows the product or the environmental forces that impact the performance of the panel better than the supplier of the panel product?
Other design factors to consider with rear ventilated cladding products are open vs. closed jointed designs and the cavity depth of the system. These will be discussed in future posts in more depth, but you should know that they can have a significant impact on the performance of the system with regards to the amount of moisture that is allowed to enter the cavity and hit the membrane as well as how designers can limit the UV degradation of the weatherproofing membrane at the panel joint of the cladding system.
Its a compelling photo visually, but not what you really want for a panel product if the end result is longevity and moisture control. And it's a great illustration of what happens when moisture isn't sufficiently controlled in a cladding system (especially with materials that aren't designed to be continually wet). And I'm sure its also probably an excellent example of someone being penny-wise but dollar foolish with the design of the system.
As part of any good cladding design, the end goal should be to control the forces that allow moisture to enter the cavity of the cladding and allow moisture to dry and exfiltrate the cavity. In the case above, it appears that edges of the panels are staying wet, causing the panels to absorb moisture and discolor.
Moisture attacks these surface edges through forces generated by gravity flow of water, capillary action and surface tension, or from an inward drive of moisture caused by negative pressure differentials in the cavity of the cladding. Also, if the cavity of the cladding is closed off or restricted from having sufficient ventilation, you may find yourself experiencing significant problems associated with condensation that occurs at the backside of your panel.
With wood and paper-based products where moisture can shorten the lifespan of your product, designing the system sufficiently will allow you to extend the serviceable life of the product. An accumulation of moisture at the backside of the panel can lead to panel bowing and warpage and ultimately failures at the fastening locations due to the stresses caused from the panel movement.
We find that one of the most underrated components of any exterior enclosure application are flashings and system terminations. You may address all of the factors mentioned above at the exterior face of the panel, but if there is a failure where it abuts a dissimilar material or where the system terminates, you could have significant moisture infiltration problems associated with the installation. That is why we feel it is critical to consider the use of products where the panel supplier is intimately involved with the design and supply of the attachment system. Who knows the product or the environmental forces that impact the performance of the panel better than the supplier of the panel product?
Other design factors to consider with rear ventilated cladding products are open vs. closed jointed designs and the cavity depth of the system. These will be discussed in future posts in more depth, but you should know that they can have a significant impact on the performance of the system with regards to the amount of moisture that is allowed to enter the cavity and hit the membrane as well as how designers can limit the UV degradation of the weatherproofing membrane at the panel joint of the cladding system.
Wednesday, July 1, 2009
Benefits of an Engineered Rainscreen Cladding System
"I'll just attach galvanized hat channels to the sheathing and fasten the panels to that! I can figure it out, it's not that difficult......" The phrase that stirs in the bellies of facade consultants yet causes Euphoria in trial litigants through out the U.S.
Common to drained and back ventilated cladding system, few suppliers of rainscreen products in the U.S. offer a system-based solution, let alone one that is engineered and tested. Is this due to the infancy of European rainscreen products in our market, or that there is a shortage of skillsets available that would encourage better practices common to most other exterior facade materials?
In the glazing industry, it is expected that the material supplier engineer, fabricate and assemble the window or curtainwall product (either fully or partially). And numerous standards are available to assess the practical performance of the glazing products (from ASTM, AAMA and NFRC). Equally common in the metal panel industry, we typically find cladding systems that are engineered for windload, seismic conditions, and thermal movements AND are tested to high performance levels for air and water infiltration.
So why is it a common practice that suppliers limit the service that they offer for their products to a "panel-only" sale? Can we assume that the construction community in the U.S. prefers to innovate rather than use solutions tested and readily available? I think this practice has been borne out of necessity on the designers part as well as a shortage of information on "best-practices" for this industry. Fortunately, forces are under foot in the U.S. to develop national standards for rainscreen cladding testing that will establish minimum standards of performance, as well as various classification levels associated with the tested performance values of the rainscreen cladding products (a standard is currently available for pressure equalized systems, and a standard for drained and back ventilated claddings is due out by the end of 2009).
The next step in the evolution of rainscreen products in the U.S. market will be to encourage the use of skilled trades for the installation of these weatherproofing products. Unlike the residential construction market where general trade contractors commonly install siding products, the commercial construction market typcially looks to specialty subcontractors for the installation of exterior cladding products. But for some reason, the installation of rainscreen cladding materials often times follows a different course. Again, I think this can be attributed to a shortage of knowledge about what comprises a quality rainscreen cladding system and what is truly required to install a cladding product that will achieve a long and serviceable life (let alone the minimum standards of performance one can expect with rainscreen cladding systems).
Our next post will discuss the parameters available with rainscreen cladding systems and the performance opportunities that can be achieved by simple design solutions.
Introduction
A rainscreen system is not a cladding material - it is a process where the substrate, waterproofing, structural support elements and cladding must be designed integrally to perform properly. It isn't just a panel screwed to a galvanized hat channel as many people espouse. In fact, it is much more sophisticated in its design approach, even when designing the connection between the panel and rivets for thermal movements (something often overlooked).
A rainscreen wall does not function properly unless the backup wall includes an effective air barrier, the wall cavity accommodates positive airflow and ventilation and the system is equipped with suitable drainage to evacuate moisture from the cavity. The ramifications of an improper design can be leaks in the exterior enclosure system that may require a partial or complete dismantling of the system to make corrections or repairs. A rainscreen system cannot be repaired by the application of sealant along breaches to the exterior face.
Alternately, the addition of wall cavities, vents and drains and air barriers, can significantly reduce heating and cooling energy losses, minimize condensation problems in summer and winter, minimize or eliminate the production of mold and reduce cladding maintenance to practically zero.
The rainscreen principle has been successfully applied to numerous types of cladding to include aluminum curtain walls, various precast wall systems, steel buildings, masonry walls and window designs. We feel that the application of the rainscreen principle offers the best overall rain penetration resistance in comparison to other methods.
This forum is a tool to educate and act as a clearing house for information on proper rainscreen design.
A rainscreen wall does not function properly unless the backup wall includes an effective air barrier, the wall cavity accommodates positive airflow and ventilation and the system is equipped with suitable drainage to evacuate moisture from the cavity. The ramifications of an improper design can be leaks in the exterior enclosure system that may require a partial or complete dismantling of the system to make corrections or repairs. A rainscreen system cannot be repaired by the application of sealant along breaches to the exterior face.
Alternately, the addition of wall cavities, vents and drains and air barriers, can significantly reduce heating and cooling energy losses, minimize condensation problems in summer and winter, minimize or eliminate the production of mold and reduce cladding maintenance to practically zero.
The rainscreen principle has been successfully applied to numerous types of cladding to include aluminum curtain walls, various precast wall systems, steel buildings, masonry walls and window designs. We feel that the application of the rainscreen principle offers the best overall rain penetration resistance in comparison to other methods.
This forum is a tool to educate and act as a clearing house for information on proper rainscreen design.
Subscribe to:
Comments (Atom)
